Power amplifier (PA) is an important component part in an electronic apparatus, as it is capable of amplifying the power of weak electric signals to satisfy the requirements of transmission and emission. The energy for amplification derives from a direct current power supply. That is to say, PA converts the direct current energy into an alternating current signal, so that power intensity of the alternating current signal meets requirements. The capability of PA to convert direct current energy into alternating current energy is referred to as efficiency of PA. The power relationship between input and output signals of PA can be divided into a linear region, a nonlinear region and a saturation region.
When the envelope of an input signal fluctuates only in the linear region, the input signal is ideally amplified. Whereas when the envelope of the input signal fluctuates to the nonlinear region, the output signal will be distorted. Such distortion manifests in the time domain to the effect that the output signal is not an ideal amplification of the input signal, and in the frequency domain to the effect that the side lobe of the spectrum of the output signal rises while the main lobe is distorted. This is not desirable.
Due to physical reasons, when envelope fluctuation of the input signal gets deeper into the nonlinear region, efficiency of PA will be much higher than that in the case of fluctuation merely in the linear region. Moreover, with the advent of novel modulation modes, dynamic range of the signal envelope becomes larger and larger, so that nonlinear distortion is inevitable, and the gist of relevant techniques rests in how to overcome such nonlinearity.
Baseband predistortion technique is an effective means in combating PA nonlinearity. The technique carries out predistortion on the baseband digital signal in advance by simulating the inverse characteristics of PA nonlinearity, so as to obtain the ideally amplified signal at the output terminal of PA.
Basic inverse characteristics of PA can be obtained through measurement, and are integrated into a predistortion module of PA as predistortion data. Due to influences of such factors as temperature, humidity and element aging, inverse characteristics of PA will be subject to change. There is hence the need to adaptively adjust the predistortion data in accordance with changes in the inverse characteristics of PA. It is necessary for the classical method (vector method) to precisely compare input data of PA with output data as fed back during operation of PA, and this brings about such problems as precise synchronization and guarantee of IQ balancing, while solutions to these problems entail considerable costs on circuits.
Predistortion methods that take scalar information such as out-band power or in-band/out-band power ratios of the output signal of the PA as the optimization objective can avoid the influence of delays of the feedback loop, and are more convenient and effective. Such methods are collectively referred to as the scalar method.
With the development of wireless communications technology, various digital modulation modes (such as 16QAM/64QAM/OFDM) with high spectral efficiencies are widely used, but these modulation modes lead to higher peak-to-average power ratio (PAPR) of the envelope of a transmitted signal, thereby putting a very high demand on the linearity of the power amplifier (PA) of the transmitter. Digital predistortion technique of the scalar method has also been given increasingly more attention.
FIG. 1 schematically illustrates a block diagram of a power amplification apparatus employing the digital predistortion technique of the scalar method. The power amplification apparatus is used for instance in a transmitter of a base station or a user terminal in a wireless communications system.
As shown in FIG. 1, having been distortion-compensated by a distortion compensator 201, the source signal from a signal source 100 is converted into an analog signal by a digital-to-analog converter 300, and is then converted by an up-converter 400 into an RF signal to be inputted into a power amplifier 500. The output signal from the power amplifier 500 is transmitted by an antenna 600. Meanwhile, a portion of the output signal from the power amplifier is fed back through coupling, is subsequently transformed to a baseband via a down-converter 205, and is sampled by an analog-to-digital converter 204.0 to obtain a digital baseband signal. This baseband signal carries therewith nonlinear characteristics of the power amplifier. An in-band/out-band power ratio calculating module 203 performs digital signal processing on the baseband signal to obtain the in-band/out-band power ratio of the output signal from the power amplifier. In accordance with the in-band/out-band power ratio, a parameter updater 202 updates parameters of a predistorter by means of an optimization algorithm to thereby achieve an adaptive predistortion process.
Very high demands are put on the analog-to-digital converter 204.0 of the feedback loop in such methods. In order to obtain the power of an out-band signal, it is necessary to perform more than three times of up-samplings on the feedback signal. However, with regard to a wideband system, the analog-to-digital converter with high sampling rate has great power consumption, is high in cost and therefore difficult for application in such devices which are sensitive to power consumption and cost (a mobile terminal, for example).    Reference Document: “Predistortion technique for high power amplifier”, US006600792.